Precise spectroscopy of metastable Li⁺ using the optical Ramsey technique in support of time dilation tests

Time dilation constitutes a crucial aspect of Lorentz invariance within special relativity and undergoes constant scrutiny through numerous Ives-Stilwell-type experiments employing the Doppler effect. In our study, we employed optical Ramsey spectroscopy on a Li + ion beam to enhance the precision of measuring the intrinsic transition frequency 2 3 S 1 −2 3 P 2 to the level of four parts in 10 10 with speed of 0.00035 c . Our findings reconciled an existing 2 MHz disparity between collinear and perpendicular laser spectroscopy. Furthermore, in conjunction with previous studies on Li + ion beams traveling at speeds of 0.064 c and 0.338 c [ Nat. Phys. 3 , 861 ( 2007 ) NPAHAX 1745-2473 10.1038/nphys778 ; Phys. Rev. Lett. 113 , 120405 ( 2014 ) PRLTAO 0031-9007 10.1103/PhysRevLett.113.120405 ], we updated the Robertson-Mansouri-Sexl parameter α ^ to be (−10.0±9.9)×10 −8 and (−2.9±2.0)×10 −8 , respectively.